1. Field of the Invention
The present invention relates to a method for driving a string of light emitting diodes (LEDs) in multiphase. More particularly, the method of the present invention drives a string of LEDs divided into groups, the groups of LEDs are turned on in sequence as the voltage applied to the string of LEDs gradually increases. The present invention also relates to a light emitting diode driver circuit for driving a string of LEDs in multiphase.
2. Description of Related Art
To drive a LED, it is necessary to provide correct voltage and/or current level. In many applications, multiple LEDs are used to provide the needed light output. LEDs can be connected either in parallel or in series or a combination of both.
In applications where the power source is periodic (such as off-line applications), the LED driver's task is to convert this time varying voltage to the correct voltage and/or current level. Such task requires circuitry commonly called ac/dc converters. These converters employ magnetic (inductor or transformer), capacitor, and/or other components, which are large in size. Certain type of capacitor such as Aluminum Electrolytic Capacitor (AEC) has short life and reduces system reliability.
Certain applications require power factor correction (PFC) where the power source's current is shaped to follow closely the power source voltage waveform. To achieve PFC, special circuits are used that often require energy hold up capacitors, more magnetic and other components.